Aluminum alloy for cathodic protection system and primary battery



United States Patent 3,368,958 ALUMINUM ALLOY FOR CATHODIC PROTEC- TIONSYSTEM AND PRIMARY BATTERY Michael J. Pryor and Douglas S. Keir, Hamden,and Philip R. Sperry, North Haven, Conn., assignors to Olin MathiesonChemical Corporation No Drawing. Continuation-impart of applicationsSer. No. 60,166, Oct. 3, 1960, Ser. No. 171,114, Feb. 5, 1962, and Ser.No. 313,445, Oct. 3, 1963. This application Mar. 30, 1965, Ser. No.444,071

12 Claims. (Cl. 204-148) ABSTRACT OF THE DISCLOSURE An aluminum alloyadvantageously utilized as sacrificial anode in cathodic protection andas anode in a primary battery, said alloy consisting essentially ofbetween 0.001 and 0.1% boron, with the tin being retained in solidsolution to the maximum degree, said maximum degree being 0.1% balanceessentially aluminum.

This application is a continuation-in-part of United States patentapplication Ser. No. 60,166 filed Oct. 3, 1960, now US. Patent3,180,728, Ser. No. 171,114 filed Feb. 5, 1962, now US. Patent 3,186,836and Ser. No. 313,445 filed Oct. 3, 1963, now US. Patent 3,241,953.

Magnesium and magnesium alloys in the form of sheet are generally usedas the anodes of electric cells or batteries adapted to utilize seawater or similar aqueous electrolytes. The cost of conventional seawater batteries utilizing magnesium and magnesium alloys has been foundto be prohibitively high except for military applications. Thisprohibitively high cost is due in part to the high price of magnesiumand also due to the difficulty in rolling the hexagonal metal down tolight gage sheet of less than 0.020 inch thickness.

Further disadvantages of magnesium and magnesium alloys for thisapplication include the fact that they generally corrode readily insaline mediums even when uncoupled. In addition, relatively low powerefficiency on the order of about 60 percent is obtained. Further, theyare accompanied by a marked hydrogen evolution problem and arecharacterized by a power output which falls with time and for whichspecial design allowances must be made.

Zinc is disadvantageous, inter alia, as it provides insufficient poweroutput to be useful anode material in this type of power cell.

The more widely used sacrificial anodes for protection of ferrousstructures against corrosion are the zinc and magnesium anodes. Aluminumalloys have not been as widely adopted for this purpose as the zinc andmagnesium anodes because they have previously produced only lowprotective currents equivalent to those generated by zinc anodes but ata much higher unit cost. Furthermore, aluminum alloys have frequentlyshown the characteristic of becoming highly polarized due to theaccumulation of insoluble corrosion products so that ultimately littleuseful protective current is delivered. Moreover, aluminum alloys whichhave been used as sacrificial anodes generally display relatively lowanodic efficiency. However, the magnesium depresses the potential ofsteel in sea water into the hydrogen evolution range and stripping ofprotective coatings from the steel can result, for example, paintcoatings. Furthermore, magnesium itself produces copious quantities ofhydrogen when it serves as an anode in sea water. This is of particularsignificance in connection With protection of sea water ballast tanks inships for which purpose magnesium anodes have been found to behazardous. Zinc is undesirable due to the low 3,368,958 Patented F eb.13,: 1 968 ice galvanic curr'entsdelivered which necessitates'the use ofa plurality of anodes in order to provide acceptable current levels.

It is therefore an object of the present invention to provide animproved aluminum alloy having a wide variety of uses, for example,which is capable of being utilized as a sacrificial anode and in animproved electric cell or battery adapted to utilize sea water or otherelectrolyte's.

It is a further object of the present invention to provide an improvedbattery as aforesaid which attains high average current density, andhigh power output for the amount of anode metal consumed.

It is a still further object of the present invention to provide animproved battery as aforesaid which is inexpensive and economical whilestill attaining excellent results.

It is a still further object of the present invention to provide animproved cathodic protection system and an improved method ofcathodically protecting a ferrous metal structure in contact with amedium corrosive thereto.

Further objects and advantages of the present invention will appearhereinafter.

In accordance with the present invention it has now been found that theforegoing objects and advantages may be readily accomplished byproviding an aluminum base alloy containing at least 90% aluminum,between 0.04 and 0.5% tin and from 0.001 to 0.1% boron, with the tinbeing retained in solid solution to the maximum degree.

The alloy of the present invention is particularly useful forfabrication into a metal anode. The anode may be advantageously utilizedin a primary electric cell containing a metal anode, a cathode and anelectrolyte in contact with said anode and cathode, for example, a drycell, an air cell, a sea water battery, etc. Still further, the anodemay be advantageously utilized in a cathodic protection systemcomprising a cathodic metal structure and the aluminous sacrificialanode of the present invention electrically connected thereto, with themetal structure and the anode being in contact with a medium corrosiveto said metal structure.

The above identified co-pending applications Ser. No. 60,166 and Ser.No. 171,114 teach that a metal anode comprising an aluminum base alloycontaining at least 0.04% tin with the tin retained in solid solution tothe maximum degree, is highly useful and advantageous and in factattains surprising advantages over previously known systems.

It has now been found that in accordance with the present inventionstill further improvements may be obtained by providing in addition tothe aluminum and tin in the alloy a specified quantity of boron. Forexample, the alloy of the present invention attains a significantimprovement in anodic efliciency. Anodic efficiency is a conventionalterm and means the ratio of the weight of consumed anode which goesdirectly into producing electric current from Faradays Law to the actualtotal weight of the anode consumed, usually expressed as a percent.Higher efficiency means less anode wastage due to local corrosion and,hence, a lower cost of cathodic protection, longer life for the anodematerial, less corrosion by-products, such as insoluble hydrated oxidesand gaseous hydrogen and more uniform galvanic current over the usefullife of the anode.

An additional advantage of the alloys of the present invention is thatthey can be readily fabricated by casting by either hot or cold rolling,and can be readily rolled to small gages and readily formed into shapes,e.g., by drawing, stamping or extruding, desirable for power cell anodesin distinction to magnesium where its hexagonal crystal lattice severelyrestricts its fabricating.

The improved aluminum alloy in the present invention contains tin in anamount from 0.04 to 0.5%, at least 90% aluminum and from 0.001 to 0.1%boron, with the tin being retained in solid solution to the maximumdegree, i.e., about 0.1% with the excess tin or a suitable thirdingredient being provided as taught in co-pending application Ser. No.60,166 to improve uniformity of corrosion and to improve anodicefiiciencies. A particularly surprising aspect of the present inventionis the consistently higher anodic efficiency in combination with goodcurrent delivery attained by the aluminum-tin-boron alloy of the presentinvention with the tin being retained in solid solution to the maximumdegree. This improvement attained by the present alloy is considerableand surprising and in fact quite important where the alloy is used as ananode.

The preferred manner of preparing this alloy is to heat the aluminum tinsample at elevated temperatures, e.g., around 540 to 640 C., with 620 C.being preferred, for a sufficient period of time to dissolve the maximumamount of tin and to redistribute excess tin or other alloying additionsin a uniform dispersion which produces maximum uniformity of attack andpower efiiciency. Generally, the heating period within the preferredtemperature range may vary between 15 minutes and 24 hours. After theheating period, the sample is cooled rapidly, for example, by immersionin a large volume of water at ambient temperatures or in the case ofthin sheet, by cooling in air. For simplicity, this treatment may betermed homogenization treatment. Homogenization within the abovetemperature range yields maximum tin in solid solu tion. Outside of thisrange the amount of tin in solid solution falls olf markedly, thusyielding poorer electrochemical characteristics.

The preferred amounts of tin are from 0.08 to 0.35 percent. In someinstances high purity aluminum may be preferred, for example, in theprimary cells; however, the present invention is not limited to the useof a high purity aluminum and in fact it is generally preferred to uselower purity aluminum, for example, containing up to 0.10% silicon andup to 0.10% iron. It should be naturally understood that the alloy ofthe present invention may contain in addition to the aluminum, tin, andboron and incidental impurities, other metallic components which may beadded to achieve particularly desirable results.

In fact, a particularly surprising aspect of the alloys of the presentinvention is that the boron addition neutralizes certain minorimpurities of the transition metal type, i.e., titanium, vanadium,manganese, zirconium and others, tying them up as boride compounds. Thetransition metal impurities generally range from traces to 0.25 percent.Thus, boron is a particularly worthwhile alloying addition since itenables the use of somewhat less pure raw materials by offsettingunintentional pickup of impurities during melting and casting as well asoffsetting naturally occurring impurities in reduction pot aluminum.

Generally, insoluble elements may be added to the alloy, i.e., elementswhich have less than 0.03% maximum solid solubility in aluminum. Thetotal amount of these insoluble elements should be no greater than 0.5%.These insoluble elements generally have no significant effect on currentoutput as they do not reduce the solid solubility of tin in aluminum,but they act as a second phase particulate cathodes and large amountsultimately reduce anodic efficiency by promoting local corrosion of theanode.

Boron may be considered as an insoluble element since it has less than0.03% maximum solid solubility in aluminum.

Soluble elements may also be added to the alloy. The soluble elementsmay be considered either lattice expanders or lattice contractors, i.e.,ternary addition elements which either expand or contract the aluminumlattice.

Generally, lattice expanders stabilize tin in retained solid solutionand permit high galvanic currents to be drawn from the alloy. Latticeexpanders may be used in an amount from about 0.001 to 8%, with typicallattice expanders and amounts thereof which may be used including:magnesium from about 0.001 to 7.0% which is particularly preferred;gallium from about 0.005 to 1.0%; zirconium from about 0.001 to 0.3%;bismuth from about 0.001 to 0.5%; indium from about 0.001 to 0.5%; andmixtures thereof. The transition metals listed above, e.g., titanium,might be necessary for grain refining or other effects in combinationwith lattice expanders which improve the galvanic characteristics.

The primary cell of the present invention may employ any cathode and anyelectrolyte. Preferably, the primary cell utilizes a consumableunpolarized cathode and a liquid electrolyte. Normally, any consumableor non-consumable and comparatively unpolarized or depolarized cathodemay be conveniently employed and optimally is a readily reducible andinsoluble metal salt or oxide, for example, a silver salt or oxide or acopper salt or oxide, a catalyzed porous electrode, such as porous metalor carbon wherein oxygen from without is continually consumed.

In the primary cell of the present invention it is preferred to utilizesolid, fused silver chloride as a cathode. Alternatively, any silversalt may be utilized as the cathode material, provided the salt is atleast as soluble as silver chloride, but sufficiently insoluble to avoiddisintegration of the cathode during operation of the cell. Among suchother cathodic materials which may be employed are silver oxide, silverchromate, silver sulfate, silver phosphate, silver acetate and silvercarbamate. Cells may be formed with cathodes of silver salts moreinsoluble than silver chloride such as silver bromide and silver iodide,but the voltage is considerably lower since the cathodic material ismuch more insoluble than the silver chloride. Exemplificative coppercompounds include preferably copper oxides.

The electrolytes which may be employed are broadly any liquid or fusedor paste electrolyte and preferably the liquid-aqueous typeelectrolytes. The electrolyte which should preferably be employedshould, in addition to being liquid at operating temperatures, be onewhich does not polarize the anode or the cathode and one free frominhibitive action on the anode. Likewise, non-consumable cathodes, suchas graphite, can be used.

The primary cell of the present invention is especially adapted toutilizing sea water as the electrolyte; however, it is apparent that thecells and batteries of the present invention will operate advantageouslyin electrolytes other than sea water, for example, any aqueous solutionof sodium chloride may be conveniently employed, such as a 3.5% aqueoussolution of sodium chloride. Similarly, other alkali metal chlorides oralkaline earth metal chlorides will be satisfactory. Other suitableelectrolytes, weak or strong, dilute or concentrated may be convenientlyemployed. Water also yields an operative cell although a considerabletime maybe required before the cell reaches its full capacity.Examplificative of the non-aqueous type electrolytes include fusedsodium chloride or potassium chloride, including low melting alkalihalide eutectics.

Naturally, the primary cell of the present invention may be prepared byany of the conventional means well known in the art. In the preparationof the primary cell of the present invention, for example, the anode andcathode material may be separated or spaced apart by any conventionalmeans, for example, thin films of a chemically stable material such asnylon may be adhered to the anode material. If the particular cell orbattery under consideration is intended to operate at a high currentdensity, the electrodes should be more closely spaced. In a cell orbattery not intended to operate at high current densities, close spacingis not required. In some cases rubber strips or tabs at the edges of'theelectrode-sheets may be employed.

The cathode material may be prepared by any of the conventional means,for example, cast sheets of substantial thickness may be employed orrolled silver chloride may be produced by suspending a body of silver,such as silver screen, in a dilute chloride solution for a timesuflicient to form a silver chloride coating of the desired thickness.Other means for preparing the cathodic material are well known in theart. It is preferred to set up a plurality of the primary cells spacedfrom one another so that individual cells are established between theplates of succeeding electrodes when immersed in an electrolyte.

It is one of the findings of the present invention that an improvedcathodic protection system may be provided comprising a cathodic metalstructure and at least one aluminous sacrificial anode electricallyconnected thereto, both the metal structure and the anode being incontact with a medium corrosive to said metal structure, said anodecomprising the above aluminum alloy of the present invention.

The anodes of the present invention can be used in cathodic protectionsystems for underground structures, such as pipe lines, foundations, andthe like. They may be used in fresh water or in saline aqueous media.They are particularly well suited for use in sea water, and providecathodic protection systems for protection of iron, such as ships hulls,ballast tanks, and commercial fishing devices, such as lobster pots,which for the first time are free from the shortcomings of previouslyused systems.

In carrying out the present invention, the sacrificial anode of the typepreviously described, is attached to a metal structure to be protected,such as, for example, a ferrous metal structure, by means of a suitableelectrical conductor, and then immersed or imbedded in the surroundingcorrosive medium, in accordance with the customary practice. The alloyanode may be of any desired shape or size, such as, for example, acylindrical piece, or a trapezoidal shaped member.

The present invention and improvements resulting therefrom will be morereadily apparent from a consideration of the following illustrativeexamples.

inches (5 mm. x 5 mm.) in cross section and-3 inches (175 mm.) length.They were. chemically cleaned and a 10- sq. cm. area was exposed in aGalvanic Cell Test sustantially as described in an article in thejournal of the Electrochemical Society, volume 105, No. 11, starting atp. 629. All determinations were carried out in 0.1 N sodium chloridesolution at 25 i0.1 C. The galvanic currents were measured continuouslyby shorting the cell through a 1 ohm resistance and continuouslyrecording the drop in potential. A second set of similar test specimenswas subjected to a modified test arrangement, Impressed Current Test,for which 1 liter of 1.0 N NaCl solution was used and aconstant currentdensity of 10 ma./cm. was maintained on the specimen, as the anode, for24 hours, employing a 10 cm? steel cathode. This current density was atleast 10 times greater than that in the Galvanic Cell Test and itapproximates conditions obtained in a larger scale galvanic test wherethe cathode area is many times larger than the anode area and where lowefiiciencies my be obtained if there is a tendency to form spongycorrosion product.

Electrochemical tests of the above two types produce a fairly largescatter. Therefore, individual test results are shown. The number ofcoulombs flowing in 48 hours in the Galvanic Cell Test is a measure ofthe ability of the anode to maintain a protecting current and dependsupon the maximum tin being in solid solution or substantially themaximum tin in solid solution with the remainder very finely dispersed.The overall percent efiiciency is the anodic eficiency. The weight ofsponge in the Impressed Current Test refers to the amount of lightlyadhering corrosion product containing entrained metallic particles whichcontributes to low efficiency.

The results are shown in the following table. Alloys A and F, withoutany boron, have low anodic efficiency in both tests. Alloy A, withdeliberately added heavy metal, titanium, has low efliciency and formsmore spongy corrosion product. Alloys B, C and D show that when boron ispresent, the efficiency markedly improves and sponge formation isreduced. Alloy 03 shows similar improvement for the alloy withoutdeliberate addition of titanium compared with Alloy F.

TABLE I Galvanic Cell Test Impressed Current Test, 10 ma./sq. cm. AlloyAlloy Composition and Method of N 0. Casting Coulombs Flowing OverallPercent Total Wt. of Sponge Overall Percent in 48 hrs. Efiiciencyma./cm. Efficiency A Al0.12 Sn-0.15 Bi0.01 Ti (DC Cast) 909, 981, 95533, 35, 35 58.8, 56. 8, 53.8 33, 34, 34 B Al0.1t2) Sn0.15 Bi-0.004Ti0.001 B (DC 890, 897, 920 53, 56, 54 4. 3, 2.7 64, 68

Cas O 11160.12) Sn0.15 Bi0.002 Ti0.022B (DO 989, 1,108, 996 50, 54, 5278, 78, 78

est D Al-0.12) Sn0.15 Bi0.008 Ti0.027 B (DO 1,106, 1,178, 1,176 58, 59,59 83, 83, 83

Cast E A1-0.12 Sn-0.15 Bi-0.01 B (TM Cast) 1,070, 1,036, 962 68, 58, 84,84, 84 F Al-0.12 Sn0.15 Bi (TM Cast) 952, 1, 329 36, 38 27. 4, 43.6 44,35

EXAMPLES This invention may be embodied in other forms or Variousaluminum base alloys were cast by conventional direct chill (DC) castingand tilt mold (TM) methods. In the casting commercial purity aluminumwas utilized containing the following impurities: iron 0.05% and silicon0.03%. As alloying additions, pure tin and bismuth were added, with thealloys containing after casting 0.12% tin and 0.15% bismuth. Inaddition, some of the alloys contained titanium and boron, with masteralloys of aluminum containing titanium and boron being utilized.

All of the alloys were given a homogenization treatment by heating to620 C. for 16 hours and Water quenching to room temperature.

All of the homogenized ingots were then given various tests to determinetheir galvanic characteristics.

Specimens were prepared from sections of the ingots by machining.Specifically, the specimens used for determining galvanic propertieswere milled to 0.197 x 0.197

carried out in other Ways without departing from the spirit or essentialcharacteristics thereof. The present embodiment is therefore to beconsidered as in all respects illustrative and not restrictive, thescope of the invention being indicated by the appended claims, and allchanges which come within the meaning and range of equivalency areintended to be embraced therein.

What is claimed is:

1. A cathodic protection system comprising a cathodic metal structureand at least one alurnino-us sacrificial anode electrically connectedthereto, both the metal structure and the anode being in contact with amedium corrosive to said metal structure, said anode being an aluminumbase alloy consisting essentially of between 0.04 and 0.5% tin, andbetween 0.001 and 0.1% boron, with the tin being retained in solidsolution to the maximum degree, said maximum degree being 0.1% balanceessenr tially aluminum.

2. The method of cathodically protecting a ferrous metal structure incontact with a medium corrosive thereto which comprises: connecting tosaid metal structure an aluminous sacrificial anode and immersing saidanode in said corrosive medium, wherein said anode is an aluminum basealloy consisting essentially of between 0.04 and 0.5 percent tin andbetween 0.001 and 0.1 percent boron, with the tin being retained insolid solution to the maximum degree, said maximum degree being 0.1%,balance essentially aluminum.

3. An aluminum base alloy consisting essentially of between 0.04 and0.5% tin, between 0.001 and 0.1% boron, with the tin being retained insolid solution to the maximum degree, said maximum degree being 0.1%,balance essentially aluminum.

4. An aluminum base alloy consisting essentially of between 0.04 and0.5% tin, between 0.001 and 0.1% boron, a material selected from thegroup consisting of between 0.001 and 8.0% of a lattice expander whichhas greater than 0.03% maximum solid solubility in aluminum, up to 0.25%of a transition metal, up to 0.10% silicon, up to 0.10% iron andmixtures thereof, with the tin being retained in solid solution to themaximum degree, said maximum degree being 0.1%, balance essentiallyaluminum.

5. An alloy according to claim 4 wherein said lattice expander isselected from the group consisting of magnesium in an amount from 0.001to 7.0%, gallium in an amount from 0.005 to 1.0%, zirconium in an amountfrom 0.001 to 0.3%, bismuth in an amount from 0.001 to 0.5%, indium inan amount from 0.001 to 0.5% and mixtures thereof.

6. In a primary electric cell containing a metal anode, a cathode, andan electrolyte in contact with said anode and said cathode, theimprovement wherein said metal anode is of an aluminum base alloyconsisting essentially of between 0.04 and 0.5% tin, between 0.001 and0.1% boron, with the tin being retained in solid solution to the maximumdegree, said maximum degree being 0.1%, balance essentially aluminum.

7. A primary electric cell according to claim 6 wherein said cathode isa consumable, unpolarized cathode and wherein said electrolyte isliquid.

8. A primary cell according to claim 7 wherein said cathode is selectedfrom the group consisting of a silver salt, a silver oxide, a coppersalt and a copper oxide, and wherein said electrolyte is an aqueouselectrolyte.

9. In a primary electric cell containing a metal anode, a cathode, andan electrolyte in contact with said anode and said cathode, theimprovement wherein said metal anode is of an aluminum base alloyconsisting essentially of between 0.04 and 0.5% tin, between 0.001 and0.1% boron, a material selected from the group consisting of between0.001 and 8.0% of a lattice expander which has greater than 0.03%maximum solid solubility in aluminum, up to 0.25% of a transition metal,up to 0.10% silicon, up to 0.10% iron, and mixtures thereof, with thetin being retained in solid solution to the maximum degree, said maximumdegree being 0.1%, balance essentially aluminum.

10. A primary cell according to claim 9 wherein said lattice expander isselected from the group consisting of magnesium in an amount from 0.001to 7.0%, gallium in an amount from 0.005 to 1.0%, zirconium in an amountfrom 0.001 to 0.3%, bismuth in an amount from 0.001 to 0.5%, indium inan amount from 0.001 to 0.5% and mixtures thereof.

11. A cathodic protection system comprising a cathodic metal structureand at least one aluminous sacrificial anode electrically connectedthereto, both the metal structure and the anode being in contact with amedium corrosive to said metal structure, said anode being an aluminumbase alloy consisting essentially of between 0.04 and 0.5% tin, between0.001 and 0.1% boron, a material selected from the group consisting ofbetween 0.001 and 8.0% of a lattice expander which has greater than0.03% maximum solid solubility in aluminum, up to 0.25% of a transitionmetal, up to 0.10% silicon, up to 0.10% iron, and mixtures thereof, withthe tin being retained in solid solution to the maximum degree, saidmaximum degree being 0.1%, balance essentially aluminum.

12. A cathodic protection system according to claim 11 wherein saidlattice expander is selected from the group consisting of magnesium inan amount from 0.001 to 7.0 percent, gallium in an amount from 0.005 to1.0 percent, zirconium in an amount from 0.001 to 0.3 percent, bismuthin an amount from 0.001 to 0.5 percent, indium in an amount from 0.001to 0.5 percent and mixtures thereof.

References Cited UNITED STATES PATENTS 3,063,832 11/1962 Snyder l383,078,191 2/1963 Maeda 75-138 3,227,644 1/1966 Rutemiller 204-1973,274,085 9/1966 Rutemiller et a1. 204-148 HOWARD S. WILLIAMS, PrimaryExaminer.

JOHN R. MACK, Examiner.

T. TUNG, Assistant Examiner.

